A surface water driving propeller assembly is a type of boat gear, in which the gear is mounted in the stern of preferably planing boats and in which the propeller assembly with its gear body projects essentially horizontally backwards (when the boat is planing) outside the stern, and which drives a propeller with an essentially straight shaft. Gears of this type are mounted in such a way, that the gear housing, when the boat is driven at speeds above a certain minimum speed, which corresponds to the lowest planing speed, is substantially parallel with the water surface and close to the water surface and in which the propeller assembly with its propeller dips into the water with only about half its height. Some propeller blades are then positioned in water, whereas other propeller blades are being ventilated in the air above the water surface. Propellers designed for this type of gear are consequently larger and/or have a larger pitch than conventional underwater-working propellers, usually at least a 15% larger diameter and pitch, because only some of the propeller blades exert a propulsion power below the water surface, and also the propellers must rotate considerably slower than conventional underwater-working propellers in order to attain the best driving conditions. Various examples of gears with surface water driving propellers are shown in European patent specification 37.690 (Arneson) or in Swedish patent accplications 8804295-7 and 8804296-5 (Thiger).
When the boat is immobile and before it has accelerated to its planing speed, all the propeller assembly and the better part of the gear body are positioned below the water surface, and a very large force from the motor is required, if the motor is to be able to accelerate the boat up to its planing speed, at which speed the propellers will be able to start working in the desired way, particularly because the propeller assembly is considerably larger and has a larger pitch than conventional underwater-working propellers.
Gears with propeller assemblies of the surface water-driving type are very different from underwater-driving propellers, i.a. since the propeller in the planing speed works in air as much as 50-70% and is considerably larger and usually has a considerably larger pitch than the corresponding underwater-working propellers and since the propeller drives the boat through a pressure force from the rear side of the propeller, while conventional underwater-working propellers propel the boat through a suction force on the front side of the propeller in substantially the same way as a sailing boat, when the wind comes ahead to port, is propelled through the suction force from the front side of the sail This is the main reason for the absence of a cavitation and a suction downwards of air from the water surface as far as a surface water-driving propeller goes, which is quite common as to conventional underwater-working propellers. Thus, it is possible as to surface water-driving propellers, already when the boat is immobile, to exert an initial force on the propeller, which corresponds to a maximum torque from the motor. In this way a boat with a surface water-driving propeller can be accelerated very strongly, and in practice such propeller assemblies, in comparison with underwater-driving propellers, prove to attain a speed increase of as much as 30-40%.
When motors, particularly Otto-engines, without turbo-charging assemblies are used, the required large initial force can often be obtained through a large gas input, but when using driving motors provided with supercharger assemblies such as turbo- or compressor-charge-assemblies, particularly supercharged Diesel-engines (turbo-Diesel engines), problems arise, which have so far been very difficult to solve. The Diesel-engines to be sure normally have a fairly small speed range and a low maximum top speed and have a relatively weak acceleration capacity from low speeds. Supercharged Diesel-engines also to be sure do not obtain their higher power range, made possible by means of the turbo-assembly, before the supercharger assembly has been connected, and this is not done before the speed is relatively high. Thus, when Diesel-engines are used, particularly supercharged Diesel-engines, in boats with gears of the above-mentioned surface water-driving type, the view has so far been that it is necessary to use an oversized engine, which is able to accelerate the boat to its planing speed within a reasonable period of time, or that it is necessary to use other, maybe expensive and complicated solutions in order to obtain a high driving motor output already from the start.
Also as far as underwater-driving propellers go, the propeller to be sure working constantly and in its entirety against water, the corresponding problem may arise but not to the same extent as in the case of surface water-driving propellers, where the propeller works against water only from the immobile condition of the boat and up to its planing speed, while the active surface of the propeller against water when the speeds are higher than the planing speed is only 40-60% of the total propeller-surface, while the remaining part of the surface works in air and substantially without any reaction requirement. As far as such underwater-driving propellers go, the propeller proportionally being smaller than surface water-driving propellers and allowed to work with a considerably higher speed than surface water-driving propellers, the above-mentioned problems could be solved by feeding air downwards to or allowing air to be sucked downwards to the propeller, in order to make the propeller "spin" and with a maintained high speed accelerate the boat to its planing speed. In certain cases this problem has also been solved by equiping the boat with an undersized propeller in order to allow a "spinning," when a cavitation and an air suction downwards take place.
Swedish document 451.449 (Brunswick Corporation), laid open to public inspection, describes a system designed to increase the acceleration of a boat by connecting between the motor and the gear a torque-boosting hydrodynamic torque converter. Such a torque converter allows a certain slippage between the pump and the turbine, often a slippage of almost 20%, which allows an acceleration of the motor, before the propeller starts to drive fully, and in this way the motor will already from the start of the acceleration cycle have a speed, which at least to some extent has approached the highest output-speed of the motor. The slippage in the torque converter is limited i.a. by the use of stationary guide rails and by the shape of the pump and turbine blades and it allows only a certain limited motor speed increase, before the successively increased hydraulic pressure in the torque converter makes the propeller drive with a substantial force. However, due to the comparatively large slippage of almost 20% between the pump and the turbine and the guide rails respectively a complete motor output on the propeller cannot be attained, and due to the risk of overheating etc. also such a slippage cannot be allowed for an extended period of time Thus, the hydrodynamic torque converter in the above-mentioned public inspection-document is according to this document designed with a lockable mechanical coupling, a so called lock-up clutch, which is connected when the motor reaches a certain predetermined speed and is disconnected when the motor speed is lower than this predetermined speed.
A device of the above-described type has some drawbacks, which make it unserviceable for gears with surface water-driving propellers and for motors of the type, which requires an almost maximum speed, before the motor output starts being transmitted to the propeller, e.g. motors having a surcharge assembly, so called turbo-motors, and this is particularly true for Diesel-engines but also for Otto-motors. In boats with such motors, for which the motor output has been calculated with regard to the maximum output at a high motor speed, said device cannot be used at all, since this high motor speed cannot be obtained before the driving force is transmitted to the propeller. Also, the device is complicated and expensive, there is a great risk of overheating and an overheating of the hydraulic medium due to the extensive slippage, special pump assemblies are required for a connection and a disconnection of the lock-up clutch, and there is a risk of slippage also in the lock-up clutch at high motor speeds and outputs.